The invention relates to a hydraulic drive mechanism for a machine element such as, for example, a punching or embossing tool executing, during a course of a machining cycle of a workpiece, a rapid feed movement toward the workpiece, thereupon with the same travel direction the working stroke, and subsequently thereto a rapid retraction movement leading back into the starting position.
A number of drive mechanisms of the aforementioned type have been proposed; however, problems are encountered in such drive mechanisms in the load-responsive switchover of the differential hydrocylinder, provided as the drive element, from rapid feed operation, wherein a large as well as small working areas of the drive piston are pressurized but wherein the maximally attainable feeding power is reduced by the ratio of the small to the large piston area, into the under-load feed operation wherein only the larger piston area is acted upon by the initial pressure of the pressure supply unit but the small piston area is pressure-relieved, which is necessary if the feeding power deployable in rapid feed operation is insufficient, for example, for penetrating the workpiece in a punching operation. When choosing a distance-dependent control of the transition from rapid feed operation to under-load feed operation, the disadvantage arises that in cases where the feeding power deployable in rapid feed operation would be adequate, and thus operation could be continued in rapid feed mode, too long cycle times must be tolerated. In order to be able to obtain a time saving in this respect, the choice is thus frequently to switch over from rapid feed operation to under-load feed operation in dependence upon the operating pressure. In other words, once the pressure in the drive pressure chambers of the differential cylinder has exceeded a threshold value, switchover is effected by an area switching valve that is pressure-controlled from rapid feed operation to under-load feed operation. However, in such a case, care must be taken that the under-load feed operation is maintained for a sufficient period of time to ensure that the pressure-controlled valve does not switch over "too early" again to rapid feed operation which could lead to undesirable vibrations and, in the extreme case, to an approximate standstill of the tool.
In order to overcome the above-mentioned problem, consideration could be given to equipping the pressure-controlled valve with an electromagnetic holding control unit in such a way that it additionally includes a control magnet maintaining the valve as soon as the latter has been switched over in dependence upon the pressure from rapid feed operation to under-load feed operation for a defined period of time in the functional position providing for under-load feed operation. However, in order to be able to utilize optimally short cycle periods, this procedure would have the consequence that the delay time during which the valve, to be switched in dependence upon the pressure, is maintained in its under-load operation function mode would have to be adapted in each case to the material thickness of the material to be worked upon. This would not only entail a considerable time consumption but also, in many cases, would result in erroneous settings which, in turn, would lead to unnecessarily long cycle times.
Therefore, it is an object of the present invention to improve a hydraulic drive mechanism of the type discussed hereinabove so that a need-responsive switchover of the drive mechanism from rapid feed operation to under-load feed operation and from the latter again to rapid feed operation and, respectively, final rapid retraction operation is made possible, independently of the thickness of a workpiece to be machined.
In accordance with advantageous features of the present invention, the hydraulic drive mechanism for a machine element such as, for example, a punching tool or an embossing tool, executing, during a course of a machining cycle of a workpiece, a rapid feed movement leading to the workpiece, thereupon, with a same travel direction, a working stroke and subsequently a rapid retraction stroke leading back into an initial position. The hydraulic drive mechanism includes a double-acting hydrocylinder as the drive element, with the double-acting hydrocylinder being constructed as a differential cylinder reciprocably accommodating a piston having a small and a large piston area defining a large and small pressure chamber. The hydraulic cylinder is adapted to provide a rapid feed operation by jointly applying an initial pressure of a pressure supply unit, and alternative pressurization and pressure relief while making it possible to control an under-load and working feed of the machine element at an increased feeding power as well as a rapid retraction operation. A pressure-controlled area switching valve is provided for switching from rapid to under-load feed operation, with the switching valve, once the drive pressure and the drive pressure chambers of the hydrocylinder exceeds a threshold value corresponding to a high percentage of, for example, 90% of a maximum output pressure of the pressure supply unit, effects switchover of the hydrocylinder mains from a differential operation to a unilateral pressure application to the large drive area of the hydrocylinder and pressure relief of the small drive area. The switching valve includes a check valve which is acted upon in an opening direction by operating pressure ambient in the small drive pressure chamber of the hydrocylinder movably defined by the small piston area of the differential piston. A closing force of a pretension closing spring urges a valve body of the check valve into a closed position by a closing force equivalent to an opening pressure of 85%-95% of the initial pressure of the pressure supply unit.
Advantageously, in accordance with further features of the present invention, the switching valve further includes a pressure-control slide valve with a valve body constructed as a stepped piston having a large and small piston step. The valve body is urged by a slightly pretension return spring into contact with the valve body of the check valve and, in a closed position of the check valve, is retained in a functional position wherein the small dry pressure chamber of the hydrocylinder is exposed to the output pressure of the pressure supply unit and, in an open position of the check valve, enters into a position wherein the small drive pressure chamber is pressure relieved. The large piston step of the piston is acted upon, on its larger piston step, by pressure ambient in the large drive pressure chamber of the hydrocylinder.
Advantageously, a ration F.sub.5 /F.sub.4 of an effective area F.sub.5 of the large piston step of the stepped piston to a cross-sectional area F.sub.4 bounded by a valve seat of the check valve, within which the valve body is acted upon in the opening direction by the pressure ambient in the small drive pressure chamber, is larger by a defined fraction A of 10% to 30% than the ratio F.sub.1 /F.sub.3 of the large piston area F.sub.1 defining the large drive pressure chamber of the hydrocylinder to the small piston area F.sub.3 defining the small drive pressure chamber.
A substantial advantage over conventional drive mechanisms is obtained with viewpoints of saving time, and due to the simplicity of the total structure also a high functional reliability is achieved, due to the accordingly provided configuration of the area switching valve in the form of a valve controlled exclusively in dependence upon the pressure wherein, by the closing force of a check valve with an adjustable force whereby the response pressure can be adjusted in a defined fashion, in combination with a multiple-way valve which produces a "hysteresis" required to prevent the drive mechanism from switching "too early" back to rapid feed operation.
According to the present invention of the switching valve includes a spherical valve member, with the spherical valve member being urged against the valve seat of the check valve by a closing spring with an adjustable bias.
In order to provide a simple structure of a directional control valve, in accordance with still further features of the present invention, a solenoid valve, fashioned as a 3/3-way valve, is provided for the directional control of the feeding and retraction motions of the piston of the drive hydrocylinder. The solenoid valve is controllable by alternative energization of control magnets into alternative functional positions wherein, in one functional position, a high pressure outlet of the pressure supply unit is connected to the large drive pressure chamber of the hydrocylinder, and in another functional position, the drive pressure chamber is pressure-relieved whereas the small drive pressure chamber can be exposed through the pressure control multiple-wave valve of the switching valve to the initial pressure of the pressure supply unit and/or relieved toward a non-pressurized tank of the pressure supply unit.
Additional objects, features, and advantages of the present invention will become more apparent from the following description of a specific embodiment when taken in connection with the accompanying drawings.